Color phosphor electroluminescent screen with filters for color cathode-ray display tubes

ABSTRACT

The screen of a three gun shadow mask type of color cathode-ray tube is formed of three interleaved sets of filters, with the filters of each set predominantly transmissive of an assigned one of the primary colors green, blue and red. The filters are circular and are of such diameter that their outer peripheral portions overlap, leaving a central section of each filter free to accept a phosphor that emits light of a wavelength for which the associated filter is predominantly transmissive. The screen, in effect, has a multiplicity of elemental picture areas grouped to form color triads and individually surrounded by a filter that attenuates substantially all visible light. Screening is accomplished with photosensitive coatings that are selectively exposed by actinic energy projected through the shadow mask. Each such exposure develops an image of one set of filters which is then developed. This process is conducted for each of the three colors to form three sets of filters with overlapping peripheral portions. Thereafter or essentially at the same time, the phosphor materials are deposited with the emission of each phosphor material properly correlated colorimetrically with the filter over which it is superposed.

United States Patent Inventor Howard G. Lange Arlington Heights, Ill.

Appl. No. 830,288

Filed June 4, 1969 Patented Mar. 9, 1971 Assignee Zenith RadioCorporation Chicago, Ill.

COLOR PHOSPHOR ELECTROLUMINESCENT SCREEN WITH FILTERS FOR COLOR CATHODE-RAY DISPLAY TUBES (B), 85, 85 (S); 117/335 (C), (CM), (P)

Primary Examiner-Roy Lake Assistant ExaminerV. LafranchiAttorney-Francis W. Crotty ABSTRACT: The screen of a three gun shadowmask type of color cathode-ray tube is formed of three interleaved setsof filters, with the filters of each set predominantly transmissive ofan assigned one of the primary colors green, blue and red. The filtersare circular and are of such diameter that their outer peripheralportions overlap, leaving a central section of eachfilter free to accepta phosphor that emits light of a wavelength for which the associatedfilter is predominantly transmissive. The screen, in effect, has amultiplicity of elemental picture areas grouped to form color triads andindividually surrounded by a filter that attenuates substantially allvisible light.

Screening is accomplished with photosensitive coatings that areselectively exposed by actinic energy projected through the shadow mask.Each such exposure develops an image of one set of filters which is thendeveloped. This process is conducted for each of the three colors toform three sets of filters with overlapping peripheral portions.Thereafter or essentially at the same time, the phosphor materials aredeposited with the emission of each phosphor material properlycorrelated colorimetrically with the filter over which it is superposed.

PATENTEU MR 9 I97! lzaw I SI/Il/l/I/III Inventor Howard G. Lun

ge y AHOZ COLOR PHOSPHOR ELECTROLUMINESCENT SCREEN WITH FILTERS FORCOLOR CATll'lODE-RAY DISPLAY TUBES RELATED PATENTS AND APPLICATIONS Thescreen structure is closely related to that described and claimed inU.S. Pat. No. 3,114,065, issued Dec. 10, 1963, in the name of Sam l-l.Kaplan and also in US. Pat. No. 3,146,368, issued on Aug. 25, 1964, inthe name of Joseph P. Fiore et a1. The process of screening is relatedto that descri and claimed in US. Pat. No. 2,959,483, issued Nov. 8,1960, in the name of Sam l-l. Kaplan and in US. Pat. No. 3,495,169issued Oct. 28, 1969 in the name of Howard G. Lange. All of theserelated patents and the related application are assigned to the assigneeof the present invention.

BACKGROUND OF THE INVENTION The present invention is directed to thescreen of, and the method of screening, a color image reproducing devicesuch as a color cathode-ray tube.

While the method concept is of general application, it is especiallyadvantageous in the fabrication of color tubes that require theelemental phosphor deposits to be smaller in size than the transparentportions of the color-selection electrode characteristically included insuch a tube. More specifically, where the tube is of the three-colorshadow mask variety, having phosphor deposits in the form of dotsarranged to constitute a multiplicity ofdot triads, it is necessary thatthe phosphor dots be. smaller than the apertures of the shadowmask ifthe tube employs either post-deflection focus or black surround. Each ofthese features is attractive.

Post-deflection focusing has the advantage of focusing the electronbeams to the end that more electrons impinge upon the elemental phosphorareas of the screen than otherwise and thus enhanced brightness isobtained. Black surround, on the other hand, is an expression used todefine a color tube in which the elemental phosphor deposits aresurrounded by a pigment or light-absorbing material, such as maganesedioxide. The use of a circumscribing light-absorbing material improvesthe contrast of the picture tube and has the further benefit of reducingthe need of, if not making totally unnecessary, the darkened orfilter-type implosion plate that is generally employed in commercialcolor television receivers. This attribute of black surroundsubstantially increases the brightness of the tube as more fullydescribed in the aforeidentified Fiore et al. patent.

Some difficulty has been experienced in screening a color tube to attainphosphor elements that are smaller in size than the transparent areas ofthe color-selection electrode. The problem is generally the same whetherthe phosphor elements are round or take the form of stripes. Therepresently is a preference to the use of phosphor dots and, accordinglythe description will continue on the assumption that the tube in processis to exhibit a mosaic type of screen formed of dot triads disposed overthe entire image area. As is well understood, each such triad includes adot of green, a dot of blue and a dot of red phosphor.

The conventional methods of screening sucha tube entail applying to theimage area a coating of a photosensitive material which is selectivelyexposed to actinic energy or radiation directed through the shadow mask.An obvious advantage derives from exposing through the shadow mask thatis ultimately to be installed as a component of the tube in process inthat this establishes the proper positions of the phosphor deposits onthe screen in relation to the apertures of the mask. In the generalcase, it can be expected that the elemental areas of the screen exposedthrough the shadow mask will have the same configuration and at leastapproximately the same size as the holes of the mask through which theexposing radiation passes. Accordingly, in an effort to have thephosphor deposits smaller in size than the apertures of the mask, it hasbeen suggested that the apertures of the mask be temporarily closed downby a filler or cladding of amaterial that may be readily removed afterthe mask with. its holes of temporarily reduced size has been used inscreening. For example, one may adopt copper or other metals as a fillerto be removed by selective etching after the screening has beenaccomplished. Experience indicates, however, that the use of a temporaryfiller leads to problems of uniformity in dot size and configuration.

In a process that lends itself more effectively to commercialproduction, the shadow mask is originally formed with holes properlysized to be used in screening so that the phosphor deposits are of theproper diameter or dimension. With this approach, the mask is reetchedafter the screening has been accomplished in order to open up or enlargethe holes to a desired size relative to. the phosphor dots that havebeen deposited on the image area of the screen. A process of this typethat has been used successfully in production is the sub ject ofapplication Ser'. No. 81 1,318, filed Mar. 28, 1969 in the name of Saml-l. Kaplan and assigned to the assignee of the present invention.

It is highly desirable to screen a color cathode-ray tube of the typeunder consideration in a process that does not require added processingsteps for the shadow mask and this objective is attainable through thepresent invention.

Accordingly, it is an object of the invention to provide a new andimproved screen structure for a color image reproducing device that isespecially attractive in practicing black surround, post-deflectionfocusing and the like.

It is another important object of the invention to provide a new processfor forming the screen of such a color image reproducing device.

It is a particular object of the invention to provide a screeningprocess for forming elemental phosphor deposits on the screen of a colorimage reproducer that are smaller than the apertures of the shadow maskwithout requiring modification: of the mask either before or after itsuse in screening.

SUMMARY or THE INVENTION A screen for a color image reproducing device,embodying the invention, comprises a substrate that is substantiallytransmissive of all light wavelengths in the visible spectrum. Aplurality of sets of image elements are disposed in an interleavedpattern over the substrate with the elements of each such. set excitableto emit light of an assigned one of a corresponding plurality of primarycolors. An attenuator for visible light wavelengths is disposed only onthe portions of the substrate that surround the image elements of eachof the sets and this attenuator comprises overlapping filters whichindividually have a relatively high transmission efficiency for light ofonly an assigned one of the primary colors and a relatively lowtransmission efficiency for light in the remainder of the visiblespectrum.

The method of the invention for forming such a screen comprises applyingto the image area a plurality of sets of filters that individually havea relatively high transmission efficiency but for light of onlyan-assigned one of the plurality of primary colors involved. Thefiltersare applied with such dimensions and such arrangement as to haveoverlapping peripheral portions which form over the screen a compositefilter for attenuating substantially all wavelengths in the visiblespectrum and patterned to surround a multiplicity of spaced elementalportions of the image area which indiviually have a high transmissionefficiency for light of an assigned primary color. A phosphor is appliedto each of those elemental portions of the image area which, whenexcited by electron bombardment, for example, emits light which,preferably, predominates at the wavelength assigned to the particularelemental area of the screen to which that phosphor has been applied.

In the simplest embodiment of the process for screening a tube which hasa shadow mask with circular apertures, the iii-- dividual filters'arecircular. They cover the multiplicity of elemental image areas which.contribute to image reproduction and individually extend beyond theassigned elemental image area to overlap with others of the filters inthe portions of the screen that surround the elemental image areas,imparting to such portions of the screen the property of attenuatinglight in the visible spectrum. Where the filter components are formed byexposing a photosensitive layer through the apertures of 5 DESCRIPTIONOF THE DRAWINGS The features of the present invention which are believedto be novel are set forth with particularity in the appended claims. Theinvention, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawing, in the several figures ofwhich like reference numerals identify like elements, and in which:

FIG. I is a fragmentary plan view of a portion of the image screen of acolor image reproducing device constructed in accordance with theinvention; and

FIG. 2 is a view taken on section line 2-2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As stated above, the inventionis useful whether the screen is made of phosphor elements that aregenerally circular or in the form of stripes but, for convenience, thenow familiar dot triad type of screen will be described. It is also tobe noted that the configuration of the tube is of no particularconsequence. The invention is equally useful for tubes having round aswell as rectangular image areas. In either case, the envelope has twoprincipal sections that are initially separated from one another whichis a convenience in screening. One of these sections is referred to asthe cap or faceplate section which is constituted of the screen or imagearea and a cicumscribing flange. The other section is conical and isconfigured as well as dimensioned at its large end to match the flangeof the faceplate so that they may ultimately be united through a fritsealing or other suitable integrating process. The smaller end of theconical section terminates in a neck which houses an assembly or clusterof three electron guns, certain of the electrodes of which areconductively connected to terminal prongs of a base applied to the endof the neck. The tube structure, aside from its screen and the method ofmaking that screen, may all be conventional and will not be consideredfurther herein.

The characteristics of the screen are illustrated in FIGS. 1 and 2 whichshows a fragment of a substrate that is substantially transmissive ofall light wavelengths in the visible spectrum. The substrate, of course,is the image area or faceplate of the picture tube and may be 100percent transmissive to visible light, or it may have some filteringattributes further to enhance contrast by having a transmissivity forvisible light of perhaps 90 percent or less. From the standpoint ofmaterial, it is a known composition of glass conventionally used informing the envelope of a color cathode-ray tube; there is nothingunique required of the material of the faceplate.

A plurality of sets of image elements are disposed in an interleavedpattern over substrate 10 characterized by the fact that the elements ofeach such set are excitable to emit light of an assigned one of acorresponding plurality of primary colors. These image elements aredeposits of various phosphor materials that are related colorimetricallyso that they add to produce white. In the conventional three coloradditive system of image reproduction currently in commercial use theprimary colors green, blue and red are employed and the correspondingphosphor deposits are designated by the letters G, B and R in thedrawing. In the representation of FIG. 1, it is clear that each suchphosphor deposit approximates a dot configuration although,specifically, it is a six-sided cusp shape figure per force of themanner in which its outline is defined as will be explained presently.

Where the tube is of the shadow mask variety in which thecolor-selection electrode is a mask with a field of circular apertures,these elemental phosphor deposits G, B and R may be said, at least to afirst approximation, to have a configuration that is generally the sameas the apertures of the mask. Since their location on the screen andspecific shape result from an exposure of the image area with actinicradiation or energy directed through the apertures of the mask, each setof image elements, be it the deposits of green, blue or red phosphor, isdistributed over the image area in the same pattern as the pattern ofapertures in the shadow mask. Moreover, the relative positions of theimage elements of the three sets are determined by the position of thesource of actinic energy in each of three exposure steps in essentiallythe same manner as conventional photoresist slurry screening processesfor depositing phosphor dots on a mosaic screen of the type underconsideration. For this reason, while the three sets of image-elementsare arranged in accordance with the same pattern, the patterns areinterleaved with one another over the substrate and collectively definethe desired multiplicity of dot triads. One such triad, for example, isidentified by reference to character 11g, llr and 11b in FIG. 1. Oneadditional characteristic of the image elements is to be noted, namely,each such element has a principal dimension that is smaller than thecorresponding dimension of the electron transparent areas or holes ofthe color-selection electrode. For example, the maximum dimensions ofimage element 11g is smaller than the hole diameter 12a of the mask 12,a fragmentary portion of which is shown in FIG. 2. In order to avoidunnecessary confusion to FIG. 1, the representation of the mask has beenomitted from that view. When the shadow mask is properly installed inoperative position in relation to image area 10, the mask holes 12a arein alignment with an assigned triad such as 11g, 111: and Mr. Theregistration of the mask holes with respect to the triads is wellunderstood and makes possible color selection by reason of the fact thatelectron beams passing through such holes selectively impact only anassigned one of the three sets of image elements. In the usual case, thetube has three electron guns each of which directs an electron beam toscreen 10 through the holes of the mask and the angular relation of thebeams to the mask is such that one beam impacts only upon green imageelements G, another impacts only upon blue image elements B, while thethird impinges solely on red image elements R. For the central portionof the screen this angular relationship is established by the geometryof the gun cluster and the necessary relation is maintained as the beamscans over the mask the screen through the influence of a dynamicconvergency' system (not shown) which need not be described since it isa structure and function that is well understood in the color imagereproducing art.

One desirable characteristic of the image elements necessary to apoat-deflection focus or black surround type of screen has already beendiscussed; that is that the effective area of the elemental phosphordeposits is smaller than the area of the holes in the aperture mask. Theexpression effective area of the elemental phosphor deposits is used tomean that portion of the phosphor deposit that contributes to imagereproduction. Obviously, any portion of a phosphor deposit that overliesa visible-light attenuator is ineffective in image synthesizing and maybe ignored. Another characteristic that is necessary if the screen is tofeature black surround is an attenuator for visible light wavelengthsdisposed on the portions of the screen or substrate that surround theseveral image elements of each of the three sets of elements and such anattenuator is indicated by the crosshatching in FIG. 1 that surroundsimage element 11g. While crosshatching has been utilized with respect tothis single element simply for the purpose of emphasis, it will beunderstood that all image elements on the screen are provided with asimilar visible light attenuator. The attenuator comprisesoverlappingfilters which individually have a relatively high transmissionefficiency for light of only an assigned one of the primary colors and arelatively low transmission efficiency for light in the remainder of thevisible spectrum. ideally, the filter components could be confinedsimply to the portions of the substrate shown in crosshatching in FIG. 1that surround the image elements and in such a case, substrate it) maybe transmissive of all primary colors and the phosphor component of eachimage element would be chosen to be predominantly emissive of the colorassigned to that particular element. in the illustrative case,specifically element Mg, the phosphor deposit would simply be greenphosphor of any known composition exhibiting the desired colorcoordinates.

in accordance with the invention, however, greatly improved processingsimplicity is achieved by using a specifically different structure inwhich each of the filter components is disposed over its associatedimage element and extends over the portion of the substrate separatingthat image element from its neighbors to constitute in this fashion onecomponent of the overlapping filters of the light attenuator. This ismost clearly represented in FIG. 2 where the filter component of thegreen image element 11g is designated 13g. it is applied directlyover'substrate and the green phosphor G is, in turn, coated over itsassociated filter component 13g. Clearly, the diameter of the filtercomponent 13g. Clearly, the diameter of the filter component 13g exceedsthe maximum dimension of the phosphor deposit G and the filter componenttherefore extends beyond the area of the image element G. In likefashion, there is shown in FIG. 2 a blue filter component 13b which isassumed to have been applied to substrate 10 after the application ofthe green filter component 13 g. As a consequence, portion 13b of theblue filter component overlaps the corresponding peripheral portion ofthe green filter component 13g. in like fashion, the red filtercomponent l3r has, a peripheral portion l3r that extends over thecontiguous portion of the green filter component 13g. Assuming the redfilter to have been the last of the three to be applied, it will haveanother peripheral portion l3r" which overlaps a portion 13b" of theblue filter component. These overlapping peripheral portions of thefilter components are represented by the crosshatching in FIG. 1. if thefilter components are properly related colorimetrically, any portion ofscreen lO where two or more of such filters overlap is essentiallyblack, that is to say, has an exceedingly low transmission efficiency inthe neighborhood of 10 to percent or less for all wavelengths in thevisible spectrum.

The described screen structure is functionally similar to that of theabove-identified F iore patent but is structurally quite different. Thereference patent shows each image element to be constituted of aphosphor deposit that is surrounded with a light-absorbing pigment,whereas in the described arrangement light attenuation in the regionsbetween image elements is achieved by a visible-light attenuatorsurrounding each image element and comprised of overlapping filterswhich individually have a relatively high transmission efficiency forlight of a particular assigned primary color but a low transmissionefficiency for light in the remainder of the visible specmm.

The materials suitable for the various filter components may be thosedescribed in Kaplan U.S. Pat. No. 3,l 14,065 and they may be appliedthrough a process similar to that claimed in its parent, namely U.S.Pat. No. 2,957,483. More specifically, vitreous color filter materialshaving a relatively low fusing temperature of the order of 430 C., whichare commercially available, are suitable for use as the filtercomponents. lllustrative designations of appropriate commerciallyavailable materials are Coming Glass No. 7570, marketed by Corning GlassWorks, and Corning Glass No. 8363. The latter is especially suitablebecause it has a thermal coefficient of expansion which matches that ofthe glass normally employed in color cathode-ray tube envelopes. Asimilar type of glass is available from Kimball Glass Company under thedesignation No. 50 solder glass which also has an acceptable fusiontemperature. Basically, these glasses are lead borate types to whichinorganic colorants are added to provide the necessary color filtercharacteristics. For example, cobalt oxide may be used as a color forthe blue filter components, copper oxide or chromium oxide for green andcadmium sulfide for red. Vitreous materials of this type areadvantageous because they are known to be compatible with high vacuumcathode-ray tubes. Additionally, they may be applied to the surface ofsubstrate 10 by photoprinting and electrostatic techniques quite similarto those employed in depositing phosphor color materials. Their lowfusing temperatures make possible affixing the filter components tosubstrate 10 in the course of normal tube processing without the needfor special processing temperatures which might introduce thepossibility of damage to the phosphor deposits. I

The methods described in U.S. Pat. No. 2,959,483 are generally useful inscreening the structure of FIG. 1 although specific changes will benecessary because the screen of the present invention includes not onlya filter component and superposed phosphor component collectivelydefining an image element as in the Kaplan patent but additionally haveextensions of the filter component into the spaces that surround itsimage element to cooperate with similar extensions of the filtercomponents of the adjacent image elements to constitute the lightattenuator or black surround.

Slurry screening with a photosensitive resist including filter materialin pulverulant form is an acceptable process. A convenient slurrycomprises an organic photosensitive material that is normally soluble inwater, such as polyvinyl alcohol sensitized with ammonium dichromate. ifsuch a slurry includes a green filter of powdered fusible material, itmay be applied in the usual manner as a coating over the entire sub-'strate 10. This provides a surface condition to the substrate that maybe altered by exposure to actinic energy, such as ultraviolet light. Inthe next process step, the shadow mask 12 is installed in its properposition relative to screen ll) and the ultraviolet source is positionedto simulate the electron gun of the tube that is intended to excite thegreen target elements or phosphor deposits. The energy source thendirects ultraviolet light through apertures 12a of the shadow mask 12 toselectively expose the coating of substrate 10 and thereby create alatent image of the set of green filter components. This exposure, ofcourse, takes place in an exposure chamber or lighthouse and the exposedportions of the polyvinyl alcohol coating are a set of circular andspaced separated areas of the screen having the same pattern arrangementas the holes of the mask. It is known that the diameter of each exposedarea is subject to control by the intensity of the light and theduration of the exposure interval.- These parameters are adjusted to theend that the latent image of the green filter components has the sizeshown in FIG. 1 by the broken construction line. This image is thendeveloped by washing substrate 10 with water to complete forming the setof green filter components 13g on substrate 10. The individual greenfilter components have, in effect, the configuration of a projection ofthe aligned hole of the shadow mask and are round since the mask hasbeen assumed to have circular apertures 12a.

A similar sequence of steps is carried out for each of the other sets ofcolor filter components. The only changes required in forming the bluefilter components 13b are the use of a photosensitive resist which has ablue filter material in powdered form rather than green and thepositioning of the exposure light sourcewhich must be modified tosimulate the electron gun intended to energize the blue image elements.In forming the red color filter components 13r, the photoresist slurryhas a red filter ingredient and the light source is positioned tosimulate the electron gun assigned to excite the red image elements.

The screen at this stage in the processing has the three interleavedpatterns of green, blue and red color filter components with their outerperipheral portions in overlapping relation to define a visible-lightattenuator surrounding elemental image areas of substrate ill and eachsuch area is covered by a single color filter that transmits essentiallyonly the color or wavelength of light that has b'eEnassigned to it. Itis now only necessary to apply the deposits of the green, blue and red.

phosphors over their assigned elemental image areas where each phosphordeposit is superposed over an associated and colorimetrically relatedfilter component. Each color phosphor may be applied through essentiallythe same steps utilized in forming the filter components. The onlysignificant difference has to do with the exposure interval, assumingthat the light source has essentially the same intensity for allexposure steps. One may, of course, use the same exposure interval inwhich case the phosphor deposit is coextensive in an area with itsassociated filter component. This certainly is not necessary and ashorter exposure time may be employed since it is only necessary thatthe phosphor be deposited over the cusped type central area of theassociated filter component. This makes clear that there is a desirabletolerance with respect to screening of the phosphor materials. So longas the minimum phosphor dot size is attained, there is no adverse effectif the deposit extends beyond the limits of the image element into theattenuator simply because the attenuator suppresses light that might beemitted from portions of the phosphor deposited within the area of theviible-light attenuator.

After the phosphor deposits have been made, the screen may be filmed andalumized in the usual fashion and the filter components will be affixedto substrate 10 by fusion either in a separate heat treatment step orduring bakeout.

The screen structure of FIG. I may also be processed through thetechniques of electrostatic screening described and claim in US. Pat.No. 3,475,169 of Howard G. Lange. In

electrostatic screening, substrate 10 is first provided with coating ofconductive material over which is superposed a coating of aphotoconductor, both preferably being organic so as to be eliminated inbakeout. The screen is then subjected to a corona discharge in order toestablish a uniform charge over the entire photoconductor. At thisjuncture, the screen is exposed to ultraviolet light through the shadowmask to establish a latent charge image of the green filter componentsand that image is developed by applying to the image area a polarizedtoner which contains fusible green filter material in powdered form. Thepolarity of the toner is determined by whether direct or reverse imagingis desired. If it be assumed that each exposed area is discharged andthe filter material is to be deposited on the discharged areas, thetoner is provided with the same polarity as the polarity of the chargeon the photoconductor. This is referred to as reverse imaging. Directimaging occurs where the polarity of the toner is opposite that of thecharge on the photoconductor in which case the toner is attracted to theportions of the photoconductor that retain a charge after exposure. TheLange application sets forth formulations of the conductive andphotoconductive layers required for the substrate and characterized bythe fact that they are organic and are easily removed during bakeout.This same general process is carried out for depositing each of thefilter components and then for depositing each of the phosphorcomponents with due regard, in each instance, to the position of thelight source, the exposure interval and the ingredients and polarity ofthe toner. Insofar as these matters are concerned, the screening processis very similar to that described above utilizing sensitized polyvinylalcohol. Of course if the three sets of filter components are appliedone after the other and then the phosphor components are sequentiallyapplied, it is necessary to recharge the photoconductive layer after theapplication of each of these various filter and phosphor components.Certain simplifications however, are possible with electrostaticscreening.

It is believed possible, for example, to apply the filter and phosphorcomponents to one set of elemental image areas concurrently. Assume thatthe photoconductor has been selectively'exposed to create a latentcharge image of the set of green filter components. If the toner used todevelop the latent image includes both the powdered form of filtermaterial and powdered phosphor material in suspension, they will settleout together and be deposited concurrently in the discharged areas ofthe screen. Desirable stratification ,may still be obtained if thefilter and phosphor materials have appropriate relative densities. Forexample, if the filter is made of a heavy material such as lead glass,it will settle out first and the phosphor will then be superposed on thefilter as preferred.

It is not necessary that the toner convey both the filter and phosphormaterials simultaneously. For example, after one latent image has beenestablished, by exposing the previously charged photoconductor, a tonercarrying filter material may be applied and the excess poured out afterthe filter material has been deposited. Sufficient residual charge willremain on the photoconductor to now permit the introduction of a secondtoner carrying phosphor material which will then deposit over thefilter, establishing the desired stratification. This has the advantageof applying both the filter and phosphor materials of one set of imageelements without the necessity of recharging the photoconductor. In thiscase, however, the filter and phosphor components of one set of imageelements is completely deposited after which the image elements of theremaining sets are similarly created. This will be a faster process thanthe first described one in which the three sets of filter componentswere first applied following which the three sets of phosphor componentswere deposited. In another variant of the process, a toner with a filtermaybe poured over the image area to develop the latent image of one setof the, filter components and, after the filter material has beendeposited, phosphor material of the appropriate color may be applied, asby spraying, over the toner to settle therethrough and deposit on top ofits associated filter component.

In any of the electrostatic screening processes in which the phosphor isdeposited directly over its associated filter without recharging of thephotoconductive layer, the filter and phosphor components of the imageelements will be coextensive in size rather than as shown in FIG. 2where the phosphor component G, for example, is smaller than its filtercomponent 13g- In such case, the overlapping filter components 13b andBr of neighboring image elements and their phosphor components overliephosphor component G as well as its filter component Preferably, thosefilter and phosphor components that do overlie illustrative phosphorcomponent G shall have a thickness, in relation to the electron velocityof the scanning electron beams during operation of the tube, toattenuate the impinging electrons to the end that they do not penetrateto phosphor component G with sufficient velocity to excite it.Otherwise, the area of the screen surrounding illustrative image elementG, 13g tends to emit green light rather than functioning in its intendedmode of a visible-light attenuator.

While electrostatic screening, including photographic techniques withexposure to ultraviolet light to create a latent charge image, has beendescribed, analogous results may be attained by scanning a layer oforganic insulating material applied to the image area with an electronbeam located at the appropriate simulated color center and having accessto the image area only through the apertures or electron transparentportions of the color-selection electrode. A screening process of thistype is described and claimed in copending application, Ser. No 773,829,filed Nov. 6, 1968, in the name of Howard G. Lange. Such a process alsopermits conveniently attaining desired relative sizes of the exposedelemental areas of the screen and the apertures of the color-selectionelectrode.

This simultaneous deposition of filter material and phosphor material,simultaneous in the sense that both materials are employed in developinga single latent charge image whether actually applied together orseriatim, has been described in the environment of a species ofblack-surround shadow-mask color cathode-ray tube where it hasespecially attractive advantages. Of course, this screening process doeshave general application and may be employed, for example, in screeningmore conventional types of color cathode-ray tubes. It is clearlyapplicable in forming screens of the type described in U.S. Pat. No. 3,]14,065 in which each elemental image area may comprise a filtercomponent and a superposed and colorimetrically correlated phosphorcomponent coextensive with its filter component but not in overlappingrelation with its neighbors. i

It is preferable that each of the sets of filter components be acontinuous coating over the associated image elements of the substratebut that is not a necessary limitation on the structure. If the filtercomponent isdiscontinuous, perhaps, having a crystalline form,acceptable results are nevertheless obtained if there is sufficientoverlap of the crystals of the various filter components to provide thedesired visible-light attenuator in the portions of the substrate thatsurround the multiplicity of elemental image areas.

A particularly attractive feature of the present invention is theelimination of any need to change the size of the mask apertures eitheras an accommodation for screening or after screening has been completed.In the process embodying the invention, the screen is formed with itsapertures of the desired final size for use as the color-selectionelectrode of the tube in process. This is a great advantage with respectto uniformity, processing complications and cost. There is the addeduniqueness of the present invention that the filter components of thevisible-light attenuator or black surround may be formed concurrentlywith the phosphor deposits and thus simplify the fabrication of thescreen. K

While a particular embodiment of the invention has been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broaderaspects, and, therefore, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

lclaim:

1. A screen for a color image reproducing device comprisa substrate thatis substantially transmissive of all light wavelengths in the visiblespectrum;

a plurality of sets of image elements disposed in an interleaved patternover said substrate with the elements of each such set excitable to emitlight of an assigned one of a corresponding plurality of primary colors;and

an attenuator for visible light wavelengths disposed only on theportions of said substrate that surround the image elements of each ofsaid sets and comprising at least a pair of overlapping filters whichindividually have a relatively high transmission efficiency for light ofonly an assigned one of said primary colors and a relatively lowtransmission efficiency for light in the remainder of the visiblespectrum.

2. A screen in accordance with claim 1 in which the image elements ofeach of said sets individually include a filter component that has arelatively high transmission efficiency for light of only the primarycolor assigned to that particular set of image elements and a phosphorcomponent which emits visi ble light in response to impinging electrons.

3. A screen in accordance with claim 2 in which:

said filter component of each image element is deposited on saidsubstrate and said phosphor component is deposited over its associatedfilter component; and

in which said filter component of each image element extends over theportions of said substrate that surround its image element to constitutea component of the overlapping filters of said attenuator.

4. A screen in accordance with claim 3 in which the phosphor componentof each image element, when excited, emits predominantly light for whichits associated filter component has relatively high transmissionefficiency.

5. A screen for a color image reproducing device that has acolor-selection electrode with a pattern of electron transparent areaswhich comprises:

a substrate that is substantially transmissive of all light wavelengthsin the visible spectrum a plurality of sets of image elements disposedin similar patterns and interleaved with one another over saidsubstrate, said image elements individually having a configurationcorresponding to that of, but a principal dimension that is smaller thanthe corresponding dimension of, the individual transparent areas of saidcolor-selection electrode and comprising a phosphor component foremitting light of an assigned one of a plurality of primary colors andalso a filter component that has a relatively high transmissionefficiency for light emitted by the associated phosphor component and arelatively low transmission efficiency for light in the remainder of thevisible spectrum; and

an attenuator for visible light wavelengths disposed only on theintervening portions of said substrate that surround said image elementsand comprising extensions of said filter components of said imageelements which project into the immediately contiguous part of saidintervening portions of said substrate in .overlapping relation withrespect to similar extensions of the filter components of the adjacentones of said image elements.

6. A screen in accordance with claim 5 in which:

said color-selection electrode is a shadow mask having a pattern ofholes through which electrons may pass; and

in which said sets of image elements are substantially in alignment withthe holes of said mask so that electrons passing through said holes mayselectively impact an image element of any of said sets of imageelements.

7. A screen in accordance with claim 6 in which:

said holes of said mask are substantially circular;

said phosphor component of .each image element is a deposit having amaximum dimension less than the diameter of said mask holes; and

said filter component is interposed between said substrate and saidphosphor component and is substantially circular with a diameterapproximately equal to the distance from the center of one image elementto the outer periphery of an adjacent image element.

8. A screen in accordance with claim 7 in which:

said screen has three sets of image elements each of which is assignedto emit light of one of the primary colors, green, blue and red;

in which the phosphor component of each image element,

in response to electron excitation, emits light of essentially only itsassigned one of said primary colors; and -in which the filter componentof each image element is predominantly transmissive of the visible lightemitted by its associated phosphor component.

2. A screen in accordance with claim 1 in which the image elements ofeach of said sets individually include a filter component that has arelatively high transmission efficiency for light of only the primarycolor assigned to that particular set of image elements and a phosphorcomponent which emits visible light in response to impinging electrons.3. A screen in accordance with claim 2 in which: said filter componentof each image element is deposited on said substrate and said phosphorcomponent is deposited over its associated filter component; and inwhich said filter component of each image element extends over theportions of said substrate that surround its image element to constitutea component of the overlapping filters of said attenuator.
 4. A screenin accordance with claim 3 in which the phosphor component of each imageelement, when excited, emits predominantly light for which itsassociated filter component has relatively high transmission efficiency.5. A screen for a color image reproducing device that has acolor-selection electrode with a pattern of electron transparent areaswhich comprises: a substrate that is substAntially transmissive of alllight wavelengths in the visible spectrum; a plurality of sets of imageelements disposed in similar patterns and interleaved with one anotherover said substrate, said image elements individually having aconfiguration corresponding to that of, but a principal dimension thatis smaller than the corresponding dimension of, the individualtransparent areas of said color-selection electrode and comprising aphosphor component for emitting light of an assigned one of a pluralityof primary colors and also a filter component that has a relatively hightransmission efficiency for light emitted by the associated phosphorcomponent and a relatively low transmission efficiency for light in theremainder of the visible spectrum; and an attenuator for visible lightwavelengths disposed only on the intervening portions of said substratethat surround said image elements and comprising extensions of saidfilter components of said image elements which project into theimmediately contiguous part of said intervening portions of saidsubstrate in overlapping relation with respect to similar extensions ofthe filter components of the adjacent ones of said image elements.
 6. Ascreen in accordance with claim 5 in which: said color-selectionelectrode is a shadow mask having a pattern of holes through whichelectrons may pass; and in which said sets of image elements aresubstantially in alignment with the holes of said mask so that electronspassing through said holes may selectively impact an image element ofany of said sets of image elements.
 7. A screen in accordance with claim6 in which: said holes of said mask are substantially circular; saidphosphor component of each image element is a deposit having a maximumdimension less than the diameter of said mask holes; and said filtercomponent is interposed between said substrate and said phosphorcomponent and is substantially circular with a diameter approximatelyequal to the distance from the center of one image element to the outerperiphery of an adjacent image element.
 8. A screen in accordance withclaim 7 in which: said screen has three sets of image elements each ofwhich is assigned to emit light of one of the primary colors, green,blue and red; in which the phosphor component of each image element, inresponse to electron excitation, emits light of essentially only itsassigned one of said primary colors; and in which the filter componentof each image element is predominantly transmissive of the visible lightemitted by its associated phosphor component.